Bit Size for CNC Cut-Out Operations

Are there reasons for preferring a 1/2-inch, 3/8-inch, or even 1/4-inch bit for cutting out parts on a CNC router? June 26, 2006

Question
What are the factors in determining the diameter of the bit for cutout being used on a CNC? I currently use a 1/2" two flute compression, which seems to work fine with my machine. I read a lot about others using 3/8" compression. Why? Is it horsepower? Is it speed of feed? Is it hold down? Is it cost? Is it a waste issue? Is it just what you were told to run from the machine manufacturer? Hoping some more seasoned CNC users can answer these.

Forum Responses
(CNC Forum)
From contributor M:
I think you will find this really boils down to two major considerations. The first is rigidity. A 1/2" bit is inherently more rigid than a 3/8" bit. Primarily because of the additional mass of the diameter. Normally larger diameter bits provide for a longer cutting edge. So typically most people would pay a little extra for a little more bit. Then came NBM (nested base manufacturing), where whole sheets are optimized and cut out on a router. Then the smaller diameter 3/8" bit began to gain in popularity, primarily because the swath left from a 3/8" bit is 25% less than that of a 1/2" bit. Less swath in the nested base world equals less vacuum loss (providing for cleaner cuts and safer operation) and to some degree more potential yield out of one sheet of goods.

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From contributor C:
Why not use a 1/4" down? We've run ours in 18mm Baltic at 500 ipm for about 2 years now. Less dust, less waste, quieter, less vacuum needed. We have very little upshear tearout, I assume because of the small size. Sure, we break a few bits from time to time, but at $14 each, I don't get too bent out of shape. The best part is we don't sharpen, we toss. So I don't have to deal with someone inputting the wrong offset. Parts are the same every time. We also use it for dadoes and whatever we are cutting. May be a little slower on some cuts, but it keeps things simple.

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From contributor G:
It really comes down to what you want to do. The 3/8" will potentially save you some waste. We see that more in the frame industry. The 1/2" tool is 50% stronger than the 3/8". Also, it has deeper fluting, allowing for better chip removal, thus extending tool life.
Faster feed speeds are also achieved with a 1/2" versus a 3/8".

Contributor C, don't throw the solid carbide tools away. Courmatt has a recycling program available. The cost of raw material has gone up considerably in the last year (the Chinese use supply and demand system). Anything we can do to reduce mining of this product helps us all. The info is on our (Courmatt's) web site, and we also pay for the freight.